Although fumbles happen for many different reasons, they are often due to cups being out of round. We have all seen cups that are badly out of round, but even cups that are just a little bit out of round can cause fumbles. Being out of round creates friction between cups and can cause fumbles when stacking fast because the cups don't separate like they should.Speed Stacks cups are designed with a specific clearance between each cup so they will separate freely, even when deformed slightly by the stackers grip. But if cups are even just a little bit out of round, the clearance is taken away so even a light grip by the stacker can cause the cups to stick.What's worse is that it all happens so fast, the stacker usually doesn't even know what caused the fumble.